Nutty was a dog (cat in the online version) who was owned by Frank Heffley when he was a child. Frank said he never saw Nutty die because he ran away to a butterfly farm. Frank was retelling this story when the Heffley Family went to brunch with Grandpa, but then Grandpa told Frank that he made up the butterfly farm story so he wouldn't have to tell him the truth. He said that the truth was, he ran over Nutty by accident with his car. After this, Frank got angry and left the brunch, taking his family with him, and leaving Grandpa with the bill. As Frank got the family home, he then drove off without telling anyone where he was going, and later on he came back with Sweetie to regain his loss.
In online version, Nutty was a cat that Grandpa left outside for a long time by mistake and the cat froze to death. It was Grandpa's birthday when he was in their house, then Frank got angry and kicked him out of the house even before he could blow the candles on his cake and eat it.